I.SUDHAKAR，V.MADHU，G.MADHUSUDHAN REDDY，K.SRINIVASA RAO*

aDepartmentofMechanical Engineering，MVGRCE，Vizianagaram，India

bDefence Metallurgical Research Laboratory，Hyderabad，India

cDepartment of Metallurgical Engineering，Andhra University，Visakhapatnam，India

Received 21 June 2014；revised 1 August 2014；accepted 12 August2014 Available online 6 November 2014

Enhancement ofwear and ballistic resistance of armour grade AA7075 alum inium alloy using friction stir processing

I.SUDHAKARa，V.MADHUb，G.MADHUSUDHAN REDDYb，K.SRINIVASA RAOc，*

aDepartmentofMechanical Engineering，MVGRCE，Vizianagaram，India

bDefence Metallurgical Research Laboratory，Hyderabad，India

cDepartment of Metallurgical Engineering，Andhra University，Visakhapatnam，India

Received 21 June 2014；revised 1 August 2014；accepted 12 August2014 Available online 6 November 2014

Industrial applications of alum inium and its alloys are restricted because of their poor tribological properties.Thermal spraying，laser surfacing，electron beam w elding are themostw idely used techniques to alter the surfacemorphology of basemetal.Prelim inary studies reveal that the coating and layering of alum inium alloyswith ceram ic particlesenhance theballistic resistance.Furthermore，among alum inium alloys，7075 alum inium alloy exhibits high strength which can be compared to that of steels and has profound applications in the designing of lightweight fortification structures and integrated protection systems.Having lim itations such as poor bond integrity，formation of detrimental phases and interfacial reaction between reinforcementand substrate using fusion route to deposithard particles paves theway to adopt friction stir processing for fabricating surface com posites using different sizes of boron carbide particles as reinforcement on armour grade 7075 alum inium alloy as matrix in the present investigation.Wear and ballistic tests were carried out to assess the performance of friction stir processed AA7075 alloy.Significant improvement inwear resistanceof friction stir processed surface composites isattributed to the change in wearmechanism from abrasion to adhesion.It has also been observed that the surfacemetalmatrix composites have shown better ballistic resistance compared to the substrate AA7075 alloy.Addition of solid lubricantMoS2has reduced the depth of penetration of the projectile to half thatof basemetal AA7075 alloy.For the first time，the friction stir processing techniquewas successfully used to improve thewear and ballistic resistances of armour grade high strength AA7075 alloy.

Armour grade aluminium alloy；Friction stir processing；Boron carbide；Molybdenum disulphide；Wear；Ballistic resistance

1.Introduction

Steel isglobally accepted asprimarily usedmaterial for the construction of m ilitary and non-m ilitary vehicles.It is attributed to the features associated w ith steel，such as high energy absorbing properties，high strength，greater notch toughness and high hardness［1-3］.Selection of suitable armourmaterials for defence applications is very crucialw ith respect to increasing mobility of the systems as well as maintaining safety.Therefore，determining the material w ith the lowest possible areal density that resists the predefined threat successfully is required in armour design studies.A number of variousmaterial systems can be considered in this perspective，especially substituting the steelsw ith lightmetal and alloys［4］.The aluminium and its alloys have profound application in the design due to their of lightweight fortification structures and integrated protection system low density，high specific strength，high specific energy absorption capability，good corrosion resistance，good thermal conductivity，less sensitivity to adiabatic shear banding and thermoplasticinstability［5］.Backman et al.［6］and Corbett et al.［7］revealed that aluminium and its alloys possess Young's modulus，strength and ductility，lowermelting point and less sensitivity to strain rate forbids it's usage as armourmaterial. Multi-layering of target or spaced structures，in extruded products or in combination w ith othermaterials，is effective measures towards improving the penetration resistance.It can be inferred that an effective combination of surface hardness to bluntor deform the projectile and subsequent dissipation of kinetic energy by supporting tough layer is primary requisite for an armour material［8-11］.Discontinuously reinforced metalm atrix composites are typically a two-component system consisting of a dispersed ceram ic phase in a m etallic matrix，which exhibits desirable mechanical properties，including high specific stiffness，high plastic flow strength，good thermal expansion，thermal stability，creep resistance，and good oxidation and corrosion resistances，suitable for automobile，aerospace，and defence industries［12］.Earlier investigations on incorporating the sandw ich of hard facing alloy and depositing a soft buttering layer in between base metal（Austenitic stainless steel）improved the ballistic immunity of steel armour welds by hindering the affect of hydrogen induced cracking（HIC）and heat affected zone（HAZ）softening introduced during weld thermal cycle used in combat vehicle construction［13-19］.Coating metallic substrateswith carbides isan effective solution in prolonging the service life of a metallic component in abrasive or erosive environments.Themechanicalperformanceof carbide coating strongly dependson the degree of dissolution of carbide in the matrix and the type of reaction layer［20］.Extensivework has been carried out for production of protective coating of silicon carbide（SiC）and tungsten carbide（WC）using surface modification techniques such as high energy lasermelt treatment，high energy electron beam irradiation and plasma transferred arc process.Among these techniques，laser melt treatment is the w idely used surface modification process. During this process，lasermelts the surface of substrate along w ith the deposited material which is usually either carbide powder（SiC，orWC）or combination of carbide powdersand a binding material（Co，Al，or Ni）［21-30］.In aforesaid techniques（liquid phase processing or fusion route），it is difficult to avoid the interfacial reaction between reinforcement and metalm atrix，and the undesirable and detrim ental phasesmay form at the surface.It also leads to defects such as pores，pin holes，shrinkage cavities，segregation，and grain coarsening［31］.Hence those cited problems can be addressed by adopting such a techniquewhich isbased on solid state.A technique that is receiving renewed attention and development all over the world is friction stir processing，which is a solid state process.Surface composite fabricated by incorporating nano sized alumina into AA6082 aluminium alloy revealed the existence of defect free interface and the perfect bonding between surface composite and substrate.Itwasalso found that thewear rate is reduced to one third of that of as received AA 6082［32］.Silicon carbide reinforced AA 2024 alloy composite has been introduced onto the surface of A 356 A l-Si alloy using friction stir processing.The obtained surface composite exhibited excellent wear resistance and metallurgical bonding with the substrate［33］.Wear characteristics of surface hybrid composites processed through friction stir processing revealed the significant improvement in wear resistance compared to that of substrate［34，35］.Keeping the afore mentioned in view，the present investigation assumes significance as studies on enhancement of wear and ballistic resistances by using friction stir processing，which have not been reported on this class of armour grade AA 7075 alum inium alloy.

Table 1Nom inal composition of AA 7075 alloy.

2.M aterial and m ethods

Basemetal AA7075-T6 aluminium alloy（substrate）of size 500×500×40mm wasused in the present investigation and its chemical composition is given in Table 1.

The surface of substratewas subjected to a depth of 3mm using friction stir processingw ith a specially designed friction stir welding machine（make-ETA Technology，Bangalore，India）.It was done with the help of two hot working tools made of high carbon steel（H13）.The first tool，a straight cylindrical friction stir tool，having shoulder?20mm w ithout pin was emp loyed to compact the pow der in the previously drilled holeson the substrate surface and to avoid scattering of the boron carbide particles during FSP.The second tool，a straight cylindrical friction stir tool（3 mm in pin length，? 6 mm in pin diameter，?20 mm in shoulder diameter），was inserted into already processed surface，i.e.，first step of friction stir processing of flat cylindrical surface.Tool rotational speed of 960 rpm，transitional speed of 50 mm/min，and plunging speed of 30 mm/min were employed.

A section cut from unprocessed and friction stir processed alloys，i.e.，surface composites were prepared form icrostructural examination.The polished surfaces were etched w ith Keller's reagent.M icrostructural examination was carried out using opticalmicroscope.The friction stir processed material was subjected to micro hardness testing employing Vickers indentation at0.3 kgf load.The pin specimens（in the form of cylindersof?4mm and 25mm in length）were subjected to dry slidewear test using Ducom pin-on-disc wear tester.The counterpartdiscwasmade of hardened alloy steelw ith surface hardnessof 65Rc.The applied load was0.5Kgf and the sliding speed was kept constant at 640 rpm.The total sliding distance for the testwas 6 km.Surface composites（targets）produced by incorporating different sizes of boron carbide particles using friction stir processing for ballistic testing are shown in Fig.1.

Ballistic testing of friction stir processed plateswas carried out as per the military standard（JIS.0108.01）.The experimental setup is given in Fig.2.The target plates were tested w ith 7.62 mm lead projectiles located at 10 m from theprojectile exit region.The striking velocity of the projectile wasmeasured using infrared lightemitting diode photovoltaic cellsbymeasuring the time interval between the interceptions caused by the projectile running across two transverse beams being spaced apart from one another at a fixed distance.The probeswere placed at 6m and 8m from the nozzle of the gun barrel，respectively.The first probe activates the timer and the second probe de-activates it.Few numbers of preliminary experiments were performed and the adjustmentsweremade to obtain the required impactvelocity of the projectile onto the target.The velocity of projectile was measured to be 830±10m/s.

Fig.1.Friction stir processed AA 7075 alloy targetsw ith B4C powdersof（a）160μm（b）60μm（c）30μm（d）30μm+MoS2.

3.Resu lts and discussion

3.1.M icrostructure

Base metal AA 7075-T6 plate exhibits elongated matrix grain morphology（pan-caked grains）along the rolling direction as evident from opticalm icrograph shown in Fig.3.

Opticalmicrograph of friction stir processed AA7075 alloy w ith boron carbide is shown in Fig.4（a）and（b）.From the m icrostructure it can be observed that there is a fine scale m icrostructural region inwhich the second phase particlesare uniform ly distributed in the matrix.After FSP，the boron carbide particles are dispersed homogenously w ithin the stir zoneand lead to the formation of defect freeand adherentB4C particles in substrate.The average size of the boron carbide particles in thestir zone isestimated significantly to besmaller than that in the as received powder.Grain refinement in the stirred region is due to dynamic recrystallization during friction stir processing.Presence of uniform ly distributed B4C particles in the composite layer leads to severe grain refinement.Besides，the grain refinement in stirred region can be attributed to restricted grain grow th as resultof grain boundary pinning by the B4C particles.

The depth of processed zone was observed to be around 3 mm，which is almost close to pin height，and the FSP zone exhibits thorough materialmixing and a more uniform distribution of intermetallic particles as compared to basemetal AA7075 alloy.Fig.4（b）shows the interfacebetween theboron carbide particlesand the substratew ithoutany defect.Itneeds to mention that the interface between the ceramic particles（B4C）and matrix plays a crucial role in determining the properties of the friction stir processed composite layer.

Fig.2.Schematic diagram of experimental setup used for ballistic testing.

Fig.3.M icrostructures of basemetal AA 7075.

3.2.Hardness testing

The hardness data of basemetal and friction stir processed alloy under differentconditions ispresented in Fig.5.There is no significant improvement in the hardnessof the friction stir processed alloy without carbide powders as compared to base metal.But a considerable improvement in the hardness of friction stir processed alloy w ith boron carbide powders was observed.Irrespective of carbide particles，the grains undergo mechanical rupture of inherentgrain boundariesdue to stirring action during friction stir processing and results in result in the formation of high angle grain boundaries.These high angle grain boundaries will impede the freemovement of dislocations and enhances the strength and hardness of surface composites.

Addition of carbide particles leads to inhomogeneous local deformation that assists the breakup of the grains.Increase inhardnessm ay be due to individual higher hardness of carbide particles and Orowan mechanism.Thismechanism explains the interaction of the dislocations w ith the non-shearable carbide particles.Dislocations make loops and bypass the particleswhich act like barriers to themovement of dislocations.This leads to the increase in strength and hardnessof the composite layer on the surface of the alloy.Grain boundary pinning by the carbide particles and associated dispersion hardening may also contribute to the improvement in the hardness of composite layer.It is evident from hardness test that there is threefold increase in the surface hardness of the AA 7075 alloy after friction surfacing w ith carbide pow der w ith finer size of 30μm.However，addition of solid lubricant molybdenum disulphide（MoS2）to boron carbide has not resulted in significant improvement in surface hardness，which may be due to the softness of the MoS2powder.

Fig.4.Opticalmicrographs of（a）the stir zone and（b）interface of surface composite during friction stir processing.

Fig.5.Comparison of Vickers hardness values（VHN）.

Fig.6.Wear rate curves of AA 7075 alum inium alloy under different conditions.

3.3.Wear testing

Wear rates of the basemetal and the friction stir processed alloy w ith surface composites under different conditions are shown in Fig.6.This reveals that the wear resistance is significantly improved w ith incorporation of carbide particles reinforcement.The improvement inwear resistance of friction stir processed carbide composites could be explained by considering its wear mechanism different from that of the substrate.It is observed that the base metal AA7075 alum inium alloy exhibited higherwear ratewhile the friction stir processed surface compositesexperienced lowerwear rate. It can be mentioned that the presence of carbide particles in thematrix of AA7075 aluminium alloy decreases the direct load betweenmatrix portion of specimen and counter disc.In fact，the load bearing capacity of the hard carbide particles decrease the direct load between the surface ofwear specimen and counter face and reduces wear rate significantly.This is attributed to localized deformation and selective removal of materials from the softermatrix，occupying the intermediate position between the mating surface and disc which acts as lubricant.From the results of hardness and wear test，it is noted that，although the hardness of boron carbide surface composite layer was only improved by around 44%and 50 VHN in comparison w ith that of base metal，which was increased from 90VHN in basemetal AA7075 to 140 VHN in friction stir processed B4C-160μm（AA7075 aluminium alloy）.Thewear resistance of the same coating issignificantly more than that of basemetal.

A detailed study of the variation of the coefficient of friction and the SEM m icrographsof theworn surface of samples were undertaken to understand the wear mechanism.At the beginning of the tests，the friction coefficientof AA7075 alloy increases to a peak value（initially at a very high rate），followed by a gradual steady state value as shown in Fig.7.

Fig.7.Variation of coefficientof friction w ith time.

Fig.8.SEM images of wear tracks of friction stir processed boron carbides（a）Basemetal AA 7075 alloy，（b-d）（160μm，60μm，30μm），and（e）friction stir processed boron carbide（30μm）+M oS2.

Fig.9.Front end of target after ballistic testing.

The increased coefficient of friction is attributed to localizing adhering of theworn debris to the surface as reported in Refs.［36，37］.It is considered that the rupture of the welded micro-parts and the occupancy of the mating surface contributed to a gradual steady value of coefficientof friction w ith time［38］.From thevariation of the coefficientof friction，it may be concluded that the predom inant wear initiation mechanism for AA7075 alloy is adhesive，which converts to abrasion at a later stage.Fig.7 shows that the friction stir processed surfacew ith boron carbide powders exhibits lower coefficient of friction compared to that of basemetal.

Fig.10.（a）Basemetal target（b）Friction stir processed AA 7075 alloy targets w ith B4C pow der 160μm（c）60μm（d）30μm（e）30μm+M oS2.

With finer carbide powder particles，the coefficient of friction decreased significantly and the result is in good agreement w ith the observed hardness data of friction stir processed surface composites.Differences in the extent of localized plastic deformation at real contactareasmay lead to the deficiencies in friction coefficient.The friction stir processed surface composites have exhibited lower friction as they are harder and undergo less plastic deformation［39］.Addition of solid lubricanthas further decreased the friction coefficientand increased thewear resistance of AA7075 alloy significantly.Thismay be attributed to the possible formation of lubricant film on the deposited coating ofboron carbidewhich reduces the tangential stresses on the surface and decrease the severity of asperitycontact.The above results are consistent w ith the scanning electron micrographs of the worn tracks of AA7075 alloy. Fig.8（a）shows the SEM micrographs of the worn tracks of AA7075 alloy.Theabrasive componentof thewearmechanism is evidenced by the ploughed grooves inside the wear tracks. Fig.8（b）shows theworn tracks observed in the surface composites.Detailed examination of thewear tracksof friction stir processed surfaces revealed the features associated w ith adhesive and abrasivemechanisms.It is evident that the extent of adhesive and abrasive wears in friction stir processed AA7075 alloy decreased due to a comparatively lower coefficient of friction and higher hardness，respectively.Fig.8（c）-（e）show s the typical SEM m icrographsof theworn surface of the friction stir processed boron carbide composite produced by FSP.The worn surface appeared comparatively smooth and had some grooves on it.It is clear from Fig.7 that the coefficient of friction of surface composites is fairly low.The low coefficient of friction indicates that themechanism of wear is predominantly adhesive in nature due to the harder（steel disk）surface scratch on the softer（pin）surface.

Fig.11.Crosssectional view s of ballistic tested basemetal target and friction stir processed AA 7075 alloy targets w ith B4C powder 60μm，30μm and 30μm+MoS2.

3.4.Ballistic behaviour

Front end appearance of targets after ballistic testing is shown in Fig.9.Ballistic testing w ith lead projectile has resulted in damage，in the form of a perforation hole with measurable depth of penetration，w idth and cracks around the hole.Cracks were generated due to transfer of high impact energy of projectileonto thesurfacewhich results in tangential shear stresses on the coating.The ballistic performance was characterized by the depth of penetration of the projectile in the targetplate，w idth of holeand total crack length.Thedepth of penetration was m easured by taking cross section of damaged targets.

After ballistic testing，themacrographs were recorded and are shown in Fig.10.W idth of the perforation hole and total crack length were measured for each target.Tested targets were cut and the macrographs of the cross sections were recorded and are shown in Fig.11.Depth of penetration was measured and ispresented in Table 2.Itgives the comparative data of the ballistic testing of base metal and friction stir processed coating w ith boron carbide pow der.

It can be seen from Table 2 that the friction stir processed AA 7075 alloy plate is able to stop the projectile whereas the uncoated base metal is perforated completely.Depth of penetration of projectile into the boron carbide surfaced base metaldecreased significantlyw ith thedecrease in particlesize，and the observed result is in agreement w ith hardness data. Further，it can be noted that the addition of solid lubricant MoS2decreased the depth of penetration of projectile and resulted in complete stopping of projectile in the thick target plate.Projectile can beseen clearly in theperforation shown in Fig.10（e）.Hence it can be concluded that friction surfacing can be successfully used to deposithard boron carbide pow der on the surface of arm our grade AA 7075 alloy.Thus themetal m atrix surface com posite layer im proved the ballistic efficiency/performance of AA7075 alloy.

Table 2Data of ballistic testing of targets.

4.Conclusions

1）Friction stir processing of AA 7075alum inum alloy resulted in fine and uniform m icrostructure consisting of carbide particles in matrix.

2）The friction stir processing of AA7075 alloy w ith boron carbide powder significantly improved thewear resistance over thatof the basemetal.Particle size of boron carbide was found to affect the wear resistance of substrate，and themaximum wear resistancewas achieved w ith particle size of 30μm.

3）Addition of solid lubricantmolybdenum disulphide into boron carbide has further improved thewear resistance of the alloy.

4）Significant improvement in ballistic performance has achieved after friction stir processing of AA7075 alloy along with boron carbide particles and molybdenum disulphide.Observed result is attributed to the improvement of hardness and wear resistance，and very low friction coefficient.

5）For the first time，the present work demonstrated successfully that the friction stir processing route is an effective strategy for enhancement of ballistic performance of AA 7075 alum inium alloy which finds w ider range of defence applications.

Acknow ledgements

The authorswould like to thank Director，Defence Metallurgical Research Laboratory，Hyderabad，India for his continued encouragementand permission to publish thiswork. Financial assistance from Armament Research Board，New Delhi，India is gratefully acknow ledged.

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E-mail addresses：sudhakar1679@gmail.com（I.SUDHAKAR），madhu_ vemuri@hotmail.com（V.MADHU），gm reddy_dm rl@yahoo.com（G. MADHUSUDHANREDDY），arunaraok@yahoo.com（K.SRINIVASA RAO）.

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